Ultrasonic, flow measuring devices are often applied in process- and automation technology. They offer simple determination of volume flow and/or mass flow in a pipeline.
Known ultrasonic, flow measuring devices frequently work according to the Doppler principle or according to the travel time difference principle. In the case of the travel time difference principle, the different travel times of ultrasonic pulses relative to the flow direction of the liquid are evaluated. For this, ultrasonic pulses are sent at a specific angle to the tube axis, both in the direction of as well as counter to the flow. From the travel time difference, the flow velocity and therewith, in the case of a known diameter of the pipeline section, the volume flow, can be determined.
In the case of the Doppler principle, ultrasonic waves with a specific frequency are coupled into the liquid, and the ultrasonic waves reflected by the liquid are evaluated. From the frequency shift between the coupled and reflected waves, the flow velocity of the liquid can likewise be determined. Reflections in the liquid occur when small air bubbles or impurities are present in the liquid, so that this principle mainly is applied in the case of contaminated liquids.
The ultrasonic waves are produced or received with the assistance of so-called ultrasonic transducers. For this, ultrasonic transducers are firmly placed in the tube wall of the relevant pipeline section. More recently, clamp-on ultrasonic, flow measuring systems are also obtainable. In the case of these systems, the ultrasonic transducers are only pressed onto the tube wall with a clamp. A large advantage of clamp-on ultrasonic, flow measuring systems is that they do not contact the measured medium and are placed on an already existing pipeline. Such systems are, for example, known from EP 686 255 131, U.S. Pat. No. 4,484,478 or U.S. Pat. No. 4,598,593.
A further ultrasonic, flow measuring device which works according to the travel time difference principle is known from U.S. Pat. No. 5,052,230. The travel time is ascertained here by means of short, ultrasonic pulses, so-called bursts.
The ultrasonic transducers are normally composed of an electromechanical transducer element, e.g. a piezoelectric element, also called piezo for short, and a coupling layer, also called a coupling wedge or, not so frequently, a lead-in element. The coupling layer is, in such case, most often manufactured from synthetic material, and the piezoelectric element is, in industrial process measurements technology, usually made from a piezoceramic. In the piezoelectric element, the ultrasonic waves are produced and, via the coupling layer, are conveyed to the tube wall and, from there, conducted into the liquid. Since the velocities of sound in liquids and synthetic materials are different, the ultrasonic waves are refracted during the transition from one medium to the other. The angle of refraction is determined in a first approximation according to Snell's law. The angle of refraction is thus dependent on the proportion of the propagation velocities in the media.
Between the piezoelectric element and the coupling layer can be arranged another coupling layer, a so-called adapting or matching layer. The adapting or matching layer simultaneously performs, in such case, the function of transmission of the ultrasonic signal and reduction of a reflection on interfaces between two materials caused by different acoustic impedances.
As a problem in the case of the ultrasound, flow measurement is that the ultrasonic transducer, which transmits and/or receives the ultrasonic signals, is composed of different materials than the measured material or the pipeline surrounding it. Each of the materials has different acoustic and physical properties (density, velocity of sound, critical temperature, thermal expansion, maximum voltages, piezoelectric effects, . . . ). For a good thermal insulation, principally metal coupling elements, or waveguides, are applied. Such sensors have relatively good, usable bandwidths. An example of a metal coupling element can be found in EP 1 332 339 A2.
Besides individual metal rods, bundles of rods have also been known for coupling of ultrasonic signals, as, for example in U.S. Pat. Nos. 5,606,297, 4,894,806, U.S. 2007/0157404, US 2007/0297739, US 2003/0097879, U.S. Pat. Nos. 4,783,997, 4,337,843. Sensors constructed thusly seem suitable for high temperature applications. Other materials are also used in order to enable high-temperature applications of ultrasonic, flow measuring devices, as U.S. Pat. No. 4,948,552 describes. Or other forms of coupling elements, as, for example, shown in U.S. Pat. No. 6,400,648. Disadvantageous in the case of these solutions is furthermore that additional matching layers for impedance matching are necessary, or a massive signal strength loss or only a small usable bandwidth must be accepted.
In U.S. Pat. No. 6,513,391, a coupling element is described, which has a cross sectional tapering of the coupling surface of the piezoelectric element and of the coupling surface facing the measured medium. This has thermal advantages. However, an impedance matching with a “matching layer” must also be performed here.